The present invention relates to a system and method which are able to purify particulate matters (hereinafter referred to as PM) and nitrogen oxides (hereinafter referred to as NOx) contained in an exhaust gas discharged from an internal combustion engine, such as a diesel engine mounted on an automobile, by means of plasma and a catalyst.
An exhaust gas, which is discharged from an internal combustion engine such as a diesel engine or lean burn engine mounted on an automobile, contains particulate matters (PM). It is known that a diesel particulate filter (hereinafter referred to as DPF) is installed downstream an internal combustion engine so as to reduce PM. A DPF, which generally includes a porous filter made of ceramics, decreases PM so as to purify an exhaust gas. This is carried out by making the exhaust gas pass through the DPF so as to separate and collect PM.
When the DPF captures a certain amount of PM, its reactivation is conducted by burning the PM with heated exhaust gas, for example, which is obtained by controlling an engine.
The exhaust gas also contains nitrogen oxides (NOx). A first method for reducing NOx is known as Hydro Carbon Selective Catalytic Reduction (HC-SCR), which provides purification of NOx by its reaction with hydrocarbons (hereinafter referred to as “HC”), using a catalyst for selective reduction of NOx. The hydrocarbons include unburned HC contained in the exhaust gas or HC contained in light oil or gasoline which is added to the exhaust gas. A second method uses a NOx adsorptive catalyst, which temporarily adsorbs NOx. This catalyst adsorbs NOx under a lean condition (oxygen excessive ambient) and releases the adsorbed NOx under a rich condition (oxygen lacking ambient), to which HC is added. A third method using a Urea Selective Catalytic Reduction (also referred to as Urea-SCR) is known. In this method, urea is added to an exhaust gas so as to produce ammonia (NH3) with a catalyst, which subsequently experiences a reaction with NOx. A fourth method uses a plasma assist catalyst for purification of NOx. Japanese Published Patent Application 2002-210366 discloses a technique related to the fourth method.
However, there has been a problem that use of DPF made of a porous filter, which causes poor flow of an exhaust gas due to greater resistance (pressure loss), does not allow efficient utilization of engine power. DPF has another problem that its reactivation requires controlling of an engine described above and its temperature possibly exceeds 1000 degrees Celsius during reactivation, which results in damage of DPF such as melting and breaking.
The first method described above has posed a problem that there is not sufficient unburned HC in an exhaust gas. The second method has a problem that it is necessary to control concentration of the oxygen in an exhaust gas and fuel consumption worsens significantly as a result of adding excessive HC relative to an amount of air breathed in. The third method has a problem that it requires apparatus for urea such as a tank and a supply device and furthermore an infrastructure for supplying urea to users. In addition, the third method has another problem that ammonia accidentally happens to escape from an automobile (ammonia slip). None of the first to fourth methods has been able to remove PM.